This invention relates to an apparatus for producing an X-ray image by using an array of X-ray detecting elements, such as an X-ray computed tomograph commonly referred to as an X-ray CT for medical use or non-destructive testing apparatus for industrial use.
The X-ray imaging apparatus is provided with a detector array composed of a plurality of X-ray detecting elements for detecting X-rays from an object being examined. For collection of data to obtain an X-ray image of the object, it has been customary to conduct an analog conversion of the current output from each of the radiation detecting elements to corresponding voltage signals. In recent years, what is called the photon counting system has been proposed, in which the pulse-like signals produced by each of the X-ray detecting elements are counted to obtain necessary image information.
The analog current-voltage conversion system has a disadvantage that the dynamic range of the system is so low that satisfactory images cannot be obtained. In particular, as the level of the output signal from the detecting elements lowers, the signal to noise ratio decreases because of remaining constant amplifier noise, so that there is a limit to detection of the signal.
The photon counting system has no such limit in a low range of radiation dose. As the radiation dose increases, however, the X-ray photons from the object under examination are more likely to be miscounted, so that there is a limit to detection of the radiation at a higher level of radiation dose.
After all, neither of the two systems is able to provide a sufficient performance to produce quality images.
Accordingly, the primary object of the invention is to eliminate the above disadvantages of the prior art systems and to provide an X-ray imaging apparatus which has a wider dynamic range than the prior art systems.
The invention will be described in detail with reference to the accompanying drawings.